Multi-color printing on web presses, whether carried out by means of the letterpress, offset or gravure methods, requires very precise registration of the impressions formed upon the paper web by the several cylinders which provide the different colors for the finished, printed web. The problem of obtaining exact registration is, of course, most severe in four-color printing, in which the web is successively printed with red (magenta), yellow, blue (cyan) and black. High quality color printing requires the closest possible registration between the four successive impressions on the web. Registration is accomplished by fine longitudinal (button-to-gear), circumferential (back and ahead), and cocking (angular about the cylinder axis) adjustment of each of the cylinders. The black is adjusted first, and the three colors are then adjusted to the black. The fineness of the adjustments can be judged by the fact that the maximum endwise or circumferential adjustment of a cylinder to achieve registry does not exceed about 0.03" (0.76 mm); and by the further fact that one complete revolution of a manual adjusting screw for cocking adjustment of the cylinder shifts the cylinder about its pivot point by only 0.0007" (0.0177 mm).
Training pressmen for operating color presses is a very complex job, and that part of it relating to register of the presses presents great difficulties because the only way of gaining practical experience in registration has been on a running press operating at its minimum speed of 500 fpm (152.4 mpm). It is quite apparent that on-press training of apprentices is a very wasteful procedure, because until acceptable registration is achieved, the entire web must be discarded.
The problem of achieving satisfactory registration of successive impressions is, of course, most severe in four-color printing; but it is also present to a lesser extent in three-color and even two-color printing.
Insofar as applicant is aware, there has never heretofore been any device upon which an apprentice could simulate the registering of the successive impressions formed by a multi-color press on a web.